Nanostructured electromagnetic metasurfaces at optical frequencies

Sammanfattning: Electromagnetic metasurfaces are broadly defined as optically thin layers
that are structured on the subwavelength scale. In general, metasurfaces
thus consist of nanoparticles, or other kinds of ”meta-atom”, arranged in
some pattern where both the individual particle sizes and the inter-particle
distances are much smaller than the wavelength. With advances in nanofabrication,
it has become feasible to precisely engineer metasurface constituent
elements to fulfil certain functions.
This thesis, focuses on the properties of metasurfaces assembled by colloidal
lithography. In contrast to most metasurfaces studied in the literature,
samples produced by colloidal lithography lack long-range periodicity.
Two different approaches to metasurface design are investigated. In appended
papers I and II, the individual elements are plasmonic gold nanoparticles
while appended paper III deals with the geometric resonances supported
in silicon nanoparticles. The investigated systems are able to convert
propagating electromagnetic fields into localized ones and vice-versa.
Hence, we can measure information about local properties in the far-field.
In paper I, the individual particles were progressively tilted with respect
to the substrate normal, resulting in an overall directional response.
This directionality was manifested in enhanced fluorescence emission in
particular directions.
In paper II, the ability of anisotropic individual particles to alter the
polarization of the incoming light beam was utilized to develop a sensing
scheme based on the detection of rotation of polarization. The change in
rotation and ellipticity of the light was shown to be sensitive to the local
refractive index around the particles. Refractometric biosensing was performed
by tracking these changes in real time.
The interaction between the incident light and geometric electric and
magnetic resonances was studied in paper III. At certain illumination conditions,
it was shown that the interference between interface reflection and
the coherent scattering from the electric and magnetic dipole resonances
gave rise to almost complete light absorption independent of polarization.
The ability to design metasurfaces with specific properties is of importance
for future applications. The results presented in this thesis contribute
to the understanding of the properties of the individual particles that compose
a metasurface and how structuring of these particles affects its overall
properties.